.. default-domain:: py
.. highlight:: py
.. currentmodule:: SpaceCenter

.. _python-api-stage:

Stage
=====


.. class:: Stage

   A single stage of a vessel. Obtain activation (burn) stages from
   ``Vessel.Stages`` / ``Vessel.StageAt``, and decouple stages from
   ``Vessel.DecoupleStages`` / ``Vessel.DecoupleStageAt``.

   .. note::

      Delta-v, thrust, TWR, specific impulse, burn time and mass members are only
      available on activation stages. On decouple stages those members throw
      InvalidOperationException because stock delta-v data does not
      apply. Thrust is reported in newtons and masses in kilograms (stock values
      are converted from kilonewtons and tonnes).

   .. attribute:: number

      The stage number (activation stage for burn stages, decouple stage otherwise).

      :Attribute: Read-only, cannot be set
      :rtype: int
   

   .. attribute:: parts

      The parts that belong to this stage.

      :Attribute: Read-only, cannot be set
      :rtype: list(:class:`Part`)
   

   .. method:: resources([cumulative = True])

      Returns a :meth:`Stage.resources` object for this stage.

      :param bool cumulative: When ``False``, only resources assigned to this stage. When ``True``, resources for this stage and all later activation or decouple stage numbers are included. On activation stages, unstaged resource containers (for example fuel tanks) are grouped with the first higher activation stage before they are detached.
      :rtype: :class:`Resources`
   

      .. note:: This is an RPC method: call ``resources()`` (optional ``cumulative`` argument).
                For activation stages, unstaged resource containers (for example fuel tanks)
                are grouped with the first higher activation stage before they detach, matching
                the C# ``Resources()`` documentation.

   .. attribute:: delta_v

      Delta-v for this stage in the current situation, in m/s.

      :Attribute: Read-only, cannot be set
      :rtype: float
   

      .. note:: Delta-v is only defined for **activation** stages (``decouple`` is ``false``).
                Calling this on a decouple stage raises an error.

   .. attribute:: vacuum_delta_v

      Vacuum delta-v for this stage, in m/s.

      :Attribute: Read-only, cannot be set
      :rtype: float
   

   .. attribute:: sea_level_delta_v

      Sea-level delta-v for this stage, in m/s.

      :Attribute: Read-only, cannot be set
      :rtype: float
   

   .. attribute:: thrust

      Thrust in the current situation, in newtons.

      :Attribute: Read-only, cannot be set
      :rtype: float
   

   .. attribute:: vacuum_thrust

      Vacuum thrust, in newtons.

      :Attribute: Read-only, cannot be set
      :rtype: float
   

   .. attribute:: sea_level_thrust

      Sea-level thrust, in newtons.

      :Attribute: Read-only, cannot be set
      :rtype: float
   

   .. attribute:: twr

      Thrust-to-weight ratio in the current situation.

      :Attribute: Read-only, cannot be set
      :rtype: float
   

   .. attribute:: vacuum_twr

      Vacuum thrust-to-weight ratio.

      :Attribute: Read-only, cannot be set
      :rtype: float
   

   .. attribute:: sea_level_twr

      Sea-level thrust-to-weight ratio.

      :Attribute: Read-only, cannot be set
      :rtype: float
   

   .. attribute:: specific_impulse

      Specific impulse in the current situation, in seconds.

      :Attribute: Read-only, cannot be set
      :rtype: float
   

   .. attribute:: vacuum_specific_impulse

      Vacuum specific impulse, in seconds.

      :Attribute: Read-only, cannot be set
      :rtype: float
   

   .. attribute:: sea_level_specific_impulse

      Sea-level specific impulse, in seconds.

      :Attribute: Read-only, cannot be set
      :rtype: float
   

   .. attribute:: burn_time

      Burn time for this stage, in seconds.

      :Attribute: Read-only, cannot be set
      :rtype: float
   

   .. attribute:: start_mass

      The mass of the vessel after the stage activates, in kg.

      :Attribute: Read-only, cannot be set
      :rtype: float
   

   .. attribute:: end_mass

      The mass of the vessel when the stage burns out, in kg.

      :Attribute: Read-only, cannot be set
      :rtype: float
   

   .. attribute:: dry_mass

      The dry mass of the vessel after the stage activates, in kg.

      :Attribute: Read-only, cannot be set
      :rtype: float
   

      .. note::

         Includes the parts of all later stages.

   .. attribute:: fuel_mass

      The fuel mass of the vessel when the stage activates, in kg.

      :Attribute: Read-only, cannot be set
      :rtype: float
   

      .. note::

         Includes the fuel of all later stages, and the fuel left in the parts the stage
         decouples. The stage burns :attr:`Stage.start_mass` minus :attr:`Stage.end_mass`.